To provide a burglar prevention device capable of making it unnecessary to perform any daily or troublesome switching operation by performing the ON/OFF control of various system controllers for executing burglar prevention or management during a key locking operation or a key unlocking operation.

A door lock is provided with a cylinder for key unlocking operation, which is formed at the outdoor side so as to be movable back and forth between a first position and a second position and an operating member normally held at the first position side so as to be operated while being linked with the backward and forward movement of the cylinder. When the cylinder is moved from the first position to the second position, the operating member is operated so that a switch arranged at the cylinder or the like can be turned on, and the ON state of the switch is held in the locked state of the door lock. When the cylinder is restored to the original position according to the key unlocking operation, the switch is turned off. The ON/OFF signals are transmitted to a burglar monitor control board, and a burglar monitor mode is turned on/off.